The present invention relates to optical fibers and, more particularly, to a method and apparatus for making an optical fiber preform using plasma outside vapor deposition process.
Various methods and techniques are known in the relevant art for making silica glass optical fiber preforms. One known method for fabricating preforms starter tubes is to heat silica and extrude it through an aperture. Another method known in the art for forming optical fiber preforms employs steps of depositing silica, either doped or undoped in accordance with desired optical properties of the finished fiber, onto a target. Several techniques for such deposition are known, including modified chemical vapor deposition (MCVD), vapor axial deposition (VAP), and outside vapor deposition (OVD). Each of these deposition methods begins with a rotating target, which can be glass, ceramic or other materials. The target can be a solid rod or a tube, with or without a reinforcing element inserted within it. Depending on the method, the target may become part of the perform and hence of the completed optical fiber, or may be removed by a later step of the fabrication process. A heat source, which can be a chemical reaction type gas burner or a plasma source, is positioned proximal to the rotating target. The position can be beneath, above, or spaced in a horizontal direction relative to the rotating axis of the target. As known in the art, the function of the heat source if to raise the temperature in the deposition zone sufficiently high for the glass-forming reactions to occur, thereby forming the desired glass particles. Depending upon which of the processes is used, the deposited glass particles are dried and sintered by another heat source, which is done by the VAD and OVD methods, or are fused into a vitreous quarts by the same heat source as was used for the deposition, as is done in the MCVD method.
For each of these deposition methods, when the target is mounted horizontally the heat source travels along with the deposition point, along the length of the target. This is done to ensure uniform deposition. If the target is a tube, the glass forming particles and materials may be deposited either on the inside surface of the tube, or on the outside surface. If the deposition is on the inside then the outside diameter remains constant, which deposition on the outside causes the outside diameter to increase.
If, on the other hand, the target is mounted vertically, the heat source is located either vertically above or laterally across. The deposition results in a substantially cylindrical product who""s diameter and length increase as deposition continues.
Examples of these and other known deposition methods appearing in the various United States Letters Patent are:
U.S. Pat. No. 3,737,292 to Keck et al. discloses a method of forming optical fibers. Multiple layers with predetermined index of refraction are formed by flame hydrolysis and deposited on the outside wall of a starting rod or member. After these layers of glass are coated on the rod the resulting hollow cylinder is heated and collapsed to form fibers.
U.S. Pat. No. 4,224,046 to Izawa et al. teaches a method for manufacturing an optical fiber preform. Two gaseous raw glass forming materials, oxygen, hydrogen and argon are jetted upwards in a burner towards a vertically mounted, rotating cylindrical start member. Soot-like glass particles are formed by flame hydrolysis and deposited on the lower end of the start member. The start member is gradually withdrawn upwards to maintain a constant spacing between the its growing end and the burner. Upon completion of the deposition, the resulting soot-like glass preform is then dried and sintered to form a transparent glass preform.
U.S. Pat. No. 4,217,027 to MacChesney et al. teaches the fabrication of preforms by what is usually referred to as the Modified Chemical Vapor Deposition (MCVD) process. In this process, a vapor stream consisting of chlorides or hydrides of silicon and germanium with oxygen is directed to the inside of a glass tube. The chemical reactions among these chemicals, which are preferentially induced by a traversing hot zone, will under proper conditions result in the formation of glass on the inner wall of the tube. The particular matter deposited on the tube is fused with each passage of the hot zone.
U.S. Pat. No. 4,412,853 to Partus discloses an MCVD process to form an optical fiber preform starter tube. The process starts with a horizontally mounted, rotating tubular target formed from glass and having a preselected composition and optical characteristics. A vapor stream is fed through the tubular target as a heat source positioned beneath the tubular target, traverses along the latter""s length. This causes reaction products of the vapor stream to be deposited on, and fuse to, the interior surface of the tubular target. The deposited material has the same index of refraction as the tubular target, but a different composition. This reference also suggests that one may achieve the same effect by an outside vapor-phase oxidation process or an outside vapor-phase axial deposition process, but does not explicitly teach how this can be done.
U.S. Pat. No. 4,741,747 to Geittner et al. is directed to the Plasma Chemical Vapor Deposition (PCVD) method of fabricating optical fibers. In this PCVD method glass layers are deposited on the inner wall of a glass tube by heating the tube to a temperature between 1100xc2x0 and 1300xc2x0 C., before passing the reactive gas mixture at a pressure between 1 and 30 hPa, and moving a plasma back and forth inside the glass tube. After the glass layers are deposited, this glass tube is collapsed to produce a solid preform. Optical fibers can be drawn from this preform.
U.S. Pat. No. 5,522,007 to Drouart et al. teaches the use of plasma deposition to build up an optical fiber preform having high hydroxyl ion concentration. In this reference, hydroxyl ions are deliberately entrained in a plasma generating gas by passing the gas through a water tank before it is introduced into one end of a plasma torch having an induction coil. The plasma torch projects molten silica particles mixed with hydroxyl ions onto a rotating substrate preform. This results in a preform having an average hydroxyl ion concentration lying in the range to 50-100 ppm deposited onto the target preform. According to Drouart et al., this technique results in optical fibers having an attenuation of 0.32 dB/km and 0.195 db/km at 1310 nm and 1550 nm, respectively.
In addition to requiring multiple processing steps to fabricate preforms, other disadvantages of the above processes include are:
1. the MCVD and PCVD processes are slower processes because of their low deposition rate;
2. the preform size is limited by the size of the deposition tube for MCVD and PCVD process; and
3. the outside vapor deposition process and the vertical axial deposition process OVD and VAD processes are based on flame hydrolysis which generates excessive amounts water and requires additional drying and sintering steps to produce high quality optical fiber preforms and require:
a. a deposition of soot particles on a target,
b. a generation of excessive amount of water as by-product, and
c. additional drying and sintering steps to produce high quality optical fiber preforms.
It is an object of the invention to provide an apparatus and method for producing an optical fiber preform having low hydroxyl content, with an increased rate of deposition, using a lower number of steps, while providing for increased preform diameter and, at the same time, increasing the quality and uniformity of the preforms.
In one embodiment of the present invention, a plasma source or torch is supported in proximity to a target rod formed from a primary material such as, for example, pure silica. The target rod is secured at each of its horizontally opposed ends and is rotated about its longitudinal axis. The plasma source deposits silica doped with a known first doping concentration. The doped silica is deposited along the length of the target rod until the latter grows to a desired diameter. The target with the doped silica is then drawn down and a thinned section is extracted for use as a secondary rod. The secondary rod has a center formed from the primary target material and an outer layer formed from the doped silica. Additional silica, having the same doping concentration, is deposited atop this secondary rod until it, too, reaches a desired diameter, and then is drawn down and a section extracted. The steps of depositing drawing down, extracting and depositing may be repeated a number of times. The result is a doped silica rod having a center formed from the primary target material with a first diameter, and an annular layer formed from the doped silica with a second outer diameter.
The doped silica rod is further processed to deposit a second silica layer. More particularly, the doped silica rod is rotated and the plasma torch moved along the length of the doped silica rod to deposit a second or outer layer of doped silica. The resulting structure may then be drawn down and a thinned section extracted, as was done for the first doped silica rod. Depending on the desired optical properties of the fiber to be produced from the preform the dopant of the second, or outer layer may be selected to either increase, or decrease, the index of refraction of the silica. In one variation of the invention the dopant concentration is varied as the outer layer is being deposited, and the result is a graded outer layer. The gradient of the dopant concentration, in accordance with known optical principles for a graded fiber preform, is generally varied from a maximum, beginning concentration level when the outer layer is first being deposited, to a minimum, end concentration level when deposition of the outer layer is almost complete.
In another variation of the invention, the dopant concentration maintained at constant as the outer layer is being deposited, and the result is a preform having a stepped index of refraction, with the index of refraction of the first silica layer typically being different from that of the second layer. The step index is attained by the second dopant concentration, used throughout the deposition of the second or outer layer, being different from the first dopant concentration used for the first silica layer.
A further embodiment of the invention forms the first doped silica layer, or the outer doped silica layer, or both, using repeated cycles of multiple deposition passes of the plasma torch, followed by a single deposition/sintering pass. The deposition/sintering pass both deposits another sublayer of the doped silica and sinters that sublayer along with the sublayers that had been deposited by the multiple deposition passes. The deposition passes do not sinter the doped silica material they deposit because the plasma torch traverses at first rate along a length of the rotating target. The first rate is selected so that the plasma flame moves at a near-maximum rate along the target, without having to impart enough heat to sinter the deposited glass. The deposition/sintering step then traverses the plasma torch at a second rate along the length of the target, the second rate being lower than the first. The second rate is selected so that the plasma flame dwells on the deposited glass sufficiently to sinter the sublayer deposited, along with all of the previously deposited sublayers.
A substantial benefit of this inventive periodic deposition/sintering feature is that the average deposition rate, in terms of grams per minute, can be approximately twice the average deposition rate if each sublayer is deposited by a deposition/sintering pass.
Still another embodiment of the invention, which may be combined with either of the above-summarized embodiments, further comprises a monitoring device for constantly detecting the diameter of the target on which the silica is being deposited, and a plasma torch controller for adjusting the distance between the extreme end of the plasma torch and the target, in response to the detected diameter. The position of the plasma torch is thereby adjusted to accommodate the increasing diameter, which maintains a constant distance between the plasma torch and the target. The present inventors have identified production improvements resulting from maintaining a constant plasma torch to target.
In a further embodiment of the present invention, a cladding layer is deposited onto the intermediate structure comprising the doped silica rod and the outer or second layer. In this embodiment, the dopant concentration of the cladding is selected according to the optical properties desired and according to the dopant concentration, and gradient, of the second layer. More particularly, if a graded second layer was deposited, the cladding layer may be formed from silica doped with the same dopant and same minimum, end concentration level used at the outermost region of the second layer. Alternatively, the cladding layer can be formed from pure silica, or even silica doped with some other dopant and at a third dopant concentration. If desired, the cladding layer may also have a graded doping.
A still further embodiment of the invention includes a stabilizer bar extending from opposite sides of the plasma torch coil. The present inventors have identified benefits with respect to deposition rate, consistency, and uniformity which are provided by the stabilizer bar in accordance with this invention.
In yet another embodiment of the invention the source gas is injected into the plasma just above a point within the plasma at which the velocity along the center axis of the plasma torch is zero. The present inventors have identified that this inventive injection arrangement reduced unwanted deposition of soot along the interior portions of the plasma torch.
An additional feature of the invention, which may be combined with any of the above-identified embodiments, comprises method steps for forming a jacketing layer over the deposited structure comprising the target, the first and the second silica layers. The jacket can be added by either further plasma deposition, or, alternatively, by providing a jacketing material over the structure and then applying heat to collapse the jacketing material into a finished preform.